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Electrically conductive and chemically stable electrode materials are required for use in both high-density dynamic random access memories (DRAMs) and ferroelectric-based memory devices (FeRAMs) that incorporate (Ba,Sr)TiO3 (BST), Pb(Zr,Ti)O3 (PZT), or SrBi2Ta2O7 (SBT) thin films as capacitor layers. Since the capacitor layer is deposited at high temperatures in oxygen ambient, the electrodes must be robust; they should not react with the substrate, not be readily oxidized, or oxidized to form electrically conductive oxides. Platinum (Pt) has been widely studied for this purpose.1-4 However, it is questionable whether the Pt electrodes can be integrated into a manufacturable process for depositing the capacitor layer. Pt films have been found to react with silicon at temperatures as low as 200 °C, forming PtSi that is easily oxidized to SiO2 by oxygen diffused through the Pt films.5 To circumvent this problem, a diffusion barrier layer, such as TiN, between Pt and polycrystalline silicon plug is commonly employed. Recently, Onishi reported that the Pt/TiN electrode tended to peel at high temperatures (∼600 °C) in oxygen ambient.6 This can be attributed to the high oxygen diffusivity of platinum, so that the barrier layer underneath the Pt film can be readily oxidized. In addition, it is difficult to etch Pt films, which adds to the difficulty in device fabrication. Therefore, alternate, chemically stable electrode materials are highly desired. The use of iridium (Ir) and iridium oxide (IrO2) electrodes is increasingly attractive due to their high conductivity and excellent thermal and chemical stability.5,7-9 Bulk Ir metal displays a resistivity of 5.3 µΩ cm at 20 °C, and IrO2 is highly conductive, with a
reported resistivity of 100 µΩ cm. Iridium films also display excellent adhesion to SiO2 and are excellent barriers to oxygen diffusion. Dielectric materials, such as BST, grown on Ir bottom electrodes exhibit high capacitance density and low leakage current.8 The oxidation of Ir, leading to the formation of IrO2, occurs only at elevated temperatures (>550 °C) in O2. During the high-temperature chemical vapor deposition (CVD) process for the growth of the dielectric or ferroelectric thin-film capacitors, the formation of IrO2 can be advantageous for limiting interdiffusion and, thereby, provide a robust, low-leakage electrode. Unlike RuO2, IrO2 is nonvolatile and, thus, stable. Ir and IrO2 films can be easily “dry” etched by chemical and plasma processes.10 Ir and IrO2 films, thus, can function as a high-temperature, stable electrode in a working device. CVD of Ir and IrO2 enables low-temperature, comformal coverage and radiation damage-free deposition as preferred for the fabrication of devices. CVD of Ir was reported previously using iridium(III) halides, iridium(III) tris(acetylacetonate) and iridium(III) tris(allyl) precursors.11 For example, Ir films were epitaxially grown onto sapphire, Si and GaAs substrates by CVD from iridium(III) tris(acetylacetonate) at deposition temperatures of 500-600 °C with oxygen as a reactive gas.12 Ir films were also deposited onto glass and quartz at atmospheric pressure at 370-550 °C in the presence of hydrogen from iridium(III) tris(acetylacetonate).13 Ir films were deposited at 100 °C from iridium(III) tris(allyl), but these compounds are extremely air-sensitive.14 Because the compounds of (βdiketonate)MIL, M ) Ag, Cu, and L ) S(C2H5)2 and 1,5cyclooctadiene (1,5-COD), have proved to be suitable precursors for the low-temperature CVD of Ag and Cu in hydrogen ambient,15,16 we believed that Ir compounds of the same empirical formulas of (β-dikeonate)IrIL, would be suitable source materials for low-temperature CVD of Ir thin films as well. In this work, we report the synthesis and characterization of (β-diketonate)IrI Lewis base adducts and the preliminary studies for their use as precursors for CVD of Ir films. Two Ir complexes, namely (hfac)IrI(1,5-COD) and (thd)IrI(1,5-COD), where hfac ) 1,1,1,5,5,5-hexafluoroacetylacetonate, thd ) 2,2,6,6-tetramethyl-3,5-heptanedionate, were synthesized through a salt-elimination reaction by mixing corresponding (β-diketonate)Na
* To whom the correspondence should be addressed. (1) Kwon, J.-H.; Yoon, S.-G. J. Electrochem. Soc. 1997, 144 (8), 2848-54. (2) Hwang, C. S.; Park, S. O.; Cho, H.-J.; Kang, C. S.; Kang, H. K.; Lee, S. I.; Lee, M. Y. Appl. Phys. Lett. 1995, 67, 2819. (3) Shimada, Y.; Inoue, A.; Nasu; T., Nagano; Y.; Matsuda, A.; Arita, K.; Uemoto, Y.; Fujii, E. Jpn. J. Appl. Phys. 1996, Part 1, 35, 4919. (4) Kawaharo, T.; Yamamuka, M.; Yuki, A.; Ono, K. Jpn. J. Appl. Phys. 1995, Part 1, 34, 5077. (5) Jeon, Y.-C.; Seon, J.-M.; Joo, J.-H.; Oh, K.-Y.; Roh, J.-S., Kim, J.-J.; Kim, D.-S. Appl. Phys. Lett. 1997, 467-469. (6) Onishi, S. IEDM Digest Technical Papers 1994, 842. (7) Hong, K.; Sun, H. J.; Yu, Y. S.; Lee, S. K. Advanced Metalization and International System for ULSI Applications in 1997; Cheung, R., Klein, J., Tsubouchi, K., Murakami, M., Kobayashi, N., Eds.; MRS, 1998; pp 735-39. (8) Balu, V.; Chen, T.; Katakam, S.; Lee, J.-H.; Lee, J. Abstract Book of the 10th International Symposium on Integrated Ferroelectrics; No. 125 P, March 1-4, 1998, Monterey, CA.
(9) Fujisawa, H.; Hyodo, S.; Nakashima, S.; Shimizu, M.; Niu, H.; Okino, H.; Abstract Book of the 10th International Symposium on Integrated Ferroelectrics, No. 23C, March 1-4, 1998, Monterey, CA. (10) Baum, T. H.; Dimeo, F. U.S. Patent (pending), 1997. (11) Hampden-Smith, M. J.; Kodas, T. T. The Chemistry of Metal CVD; Kodas, T. T., Hampden-Smith, M. J., Eds.; VCH: New York, 1994; Chapter 8, pp 398-9. (12) Vargas, R.; Goto, T.; Zhang, W.; Hirai, T. Appl. Phys. Lett. 1994, 65 (9), 1094-6. (13) Gelfond, N.; Tuzikov, F. V.; Igumenov, I. K. Thin Solid Films 1993, 227 (2) 144-52. (14) Kaesz, H. D.; Williams, R. S.; Hicks, R. F.; Chen, Y.-J. A.; Xue, Z.; Xu, D.; Shuh, D. K.; Thridandam, H. Mater. Res. Soc. Symp. Proc. 1989, 131, 395-400. (15) Xu, C.; Hampdem-Smith, M. J.; Kodas, T. T. Adv. Mater. 1994, 6, 746-8. (16) Hampden-Smith, M. J.; Kodas, T. T. In The Chemistry of Metal CVD; Kodas, T. T., Hampden-Smith, M. J., Eds.; VCH: New York, 1994; Chapter 5, pp 267-8.
New Precursors for Chemical Vapor Deposition of Iridium Chongying Xu and Thomas H. Baum* Advanced Delivery and Chemical Systems, Ltd., 7 Commerce Drive, Danbury, Connecticut 06810
Arnold L. Rheingold Department of Chemistry, University of Delaware, Newark, Delaware 19716 Received May 13, 1998 Revised Manuscript Received July 20, 1998
S0897-4756(98)00346-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/15/1998
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and chloro-1,5-cyclooctadiene iridium(I) dimer in diethyl ether at room temperature.17 The ether was evaporated under vacuum after completion of the reaction, and the solid residue was extracted with dry pentane. After filtration, removal of pentane in vacuo gave solid products that gave satisfactory elemental analysis results without further purification. The (hfac)IrI(1,5COD) is a dark brown solid while the (thd)IrI(1,5-COD) is a bright yellow solid. Both complexes were characterized by solution 1H and 13C NMR and solid-state Fourier transform (FT) IR. The spectroscopic data were consistent with their molecular formulas. The complexes are air-stable,18 which further simplifies the handling and operation of a CVD process. The solid-state molecular structure of (hfac)IrI(1,5COD) was determined by single-crystal X-ray diffraction.19 Single crystals of (hfac)IrI(1,5-COD) were grown from a hexane solution by slow evaporation of the solvent at room temperature. A suitably sized crystal was selected for single crystal X-ray diffraction studies. A summary of the crystal data and experimental details is presented in Table 1. There are two identical, but crystallographically independent, molecules in the structure. Shown in Figure 1 is an ORTEP plot of the molecular structure of (hfac)IrI(1,5-COD). The fluorine atoms in one of the trifluoromethyl groups of the molecule were disordered. It is monomeric with a nearly perfectly square planar geometry around iridium metal (based on the CdC centroids). The maximum deviation from the planarity is ca. 0.02 Å and the angles are within 2° of 90° or 180°. The volatility and thermal stability of (hfac)IrI(1,5COD) and (thd)IrI(1,5-COD) were qualitatively examined by thermal analysis and vacuum sublimation. Both compounds were analyzed by simultaneous ther(17) Preparation of (hfac)IrI(1,5-COD): The general reaction was carried out under a steady flow of nitrogen using standard Schlenk techniques. Chloro-1,5-cyclooctadieneiridium(I) dimer (1.0 g, 1.5 mmol) and (hfac)Na (0.70 g, 3.0 mmol) were suspended in 10 mL of diethyl ether in a Schlenk flask. The mixture was stirred magnetically at room temperature. The solution became dark and white precipitation was observed. The mixture was stirred for 4 h at room temperature. Then the ether was evaporated under vacuum and a black solid residue was obtained; 20 mL of dry pentane was used to extract the product from the residue. After filtration, removal of the pentane filtrate in vacuo gave 1.3 g (∼85%) of dark brown solid, which can be further purified by sublimation at ∼60 °C under 50 mTorr. Characterization data: Anal. Calcd for C13H13O2F6Ir: C, 30.77; H, 2.58. Found: C, 30.91; H, 2.58. FT-IR (KBr pellet, cm-1): 2954 (m), 2843 (m), 1595 (s), 1555 (m), 1444 (s), 1350 (m), 1260 (s), 1200 (s), 1154 (s), 1102 (m), 982 (m), 817 (m), 689 (m). 1H NMR (C6D6): δ 1.39 (m, 4H, 2 × -CH2-), 1.96 (m, 4H, 2 × -CH2-), 4.21 (m, 4H, 2 × -CHdCH-), 6.02 (s, 1H, -CH- of hfac). 13C NMR (C6D6): δ 30.9 (s, -CH2- of 1,5-COD), 62.0 (s, -CHd CH- of 1,5-COD), 93.0 (s, -CH- of hfac), 118.8 (q, CF3), 174.1 (q, CdO). (thd)IrI(1,5-COD) was prepared similarly from (thd)Na and [ClIrI(1,5-COD)]2. Characterization data: Anal. Calcd for C19H31O2Ir: C, 47.18; H, 6.46. Found: C, 47.55; H, 6.58. FT-IR (KBr pellet, cm-1): 2965 (m), 2874 (m), 1538 s), 1499 (s), 1447 (m), 1375 (s), 1356 (s), 1219 (m), 1134 (m), 977 (m), 877 (m), 800 (m), 644 (m); 1H NMR (C6D6): δ 1.11 (s, 18H, 6 × CH3), 1.60 (m, 4H, 2 × -CH2-), 2.19 (m, 4H, 2 × -CH2-), 4.25 (m, 4H, 2 × -CHdCH-), 5.92 (s, 1H, -CH- of thd); 13C NMR (C D ): δ 28.9 (s, CH ), 31.9 (s, -CH- of 1,5-COD), 41.6 [s, 6 6 3 C(CH3)3 ], 59.2 (s, -CHdCH- of 1,5-COD), 92.2 (s, -CH- of thd), 196.8 (s, CdO). (18) No NMR spectral changes were noted in the Ir precursors after 7 days (168 h) of ambient air exposure. (19) The diffraction data were collected on a Siemens P4 diffractometer equipped with a SMART CCD detector. Semiempirical corrections for absorption were applied to the data. The structure was solved using direct methods, completed by subsequent difference Fourier synthesis and refined by full-matrix, least-squares procedures. All software and sources of the scattering factors are contained in the SHELXTL (version 5.03) program library (G. Sheldrick, Siemens XRD, Madison, WI).
Communications Table 1. Crystal, Data Collection, and Refinement Parameters for (hfac)IrI(1,5-COD) empirical formula FW cryst syst space group a, Å b, Å c, Å V, Å3 Z ρcalc, g/cm3 cryst color and habit abs coeff, mm-1 temp, K 2θ scan range, deg no. of rflns collected no. of indep rflns transmission (min, max) No/Nv GOF R(F),a % Rw(F2),a % diff peak (min, max) e/Å3
C13H13F6IrO4 507.43 orthorhombic Pbca 17.6226(2) 16.3050(3) 20.3016(2) 5833.4(2) 16 2.311 dark red block 9.225 198(2) 4.0-56.7 22300 6824 0.745, 0.888 15.0 1.074 2.79 7.35 0.914, -0.878
a Quantity minimized ) R(F) ) ∑||F | - |F ||/∑|F |; Rw(F2) ) o c o [∑[w(Fo2 - Fc2)2/∑[(wFo2)2]]1/2.
Figure 1. Molecular structure of (hfac)IrI(1,5-COD) showing disordered fluorine atoms. Selected bond lengths (Å) and angles (deg) are given as follows: Ir-O(1) ) 2.064(3), Ir-O(2) ) 2.067(3), Ir-C(5) ) 2.090(5), Ir-C(6) ) 2.108(5), Ir-C(1) ) 2.100(5), Ir-C(2) ) 2.102(5); O(1)-Ir-O(2) ) 90.58(13), O(1)Ir-C(2) ) 91.0(2), O(1)-Ir-C(1) ) 88.7(2), O(2)-Ir-C(5) ) 89.8(2), and O(2)-Ir-C(6) ) 92.3(2).
mogravimetric analysis (TGA) and differential scanning calorimetry (DSC) with a heating rate of 10 °C/min using Ar as a purge gas (25 mL/min). (hfac)IrI(1,5-COD) melted at 120 °C and was completely transported by vaporization below 230 °C at atmospheric pressure.20 In the case of (thd)IrI(1,5-COD), a melting endotherm was observed at 179 °C and a 6.3% residual weight was observed at temperatures >294 °C, which is far below that calculated for the molecular iridium content of 39.8%. This discrepancy is due to weight loss from vaporization prior to pyrolytic decomposition at the elevated temperatures. The vaporization of each complex under vacuum was also studied. All of these complexes sublimed intact at 60-70 °C at 50 mTorr. The chemical vapor deposition of Ir films was studied using (hfac)IrI(1,5-COD) as the precursor. Blanket (20) The thermal analysis data of (hfac)IrI(1,5-COD) and scanning electron micrograph of the deposited Ir film from this precursor are available as Supporting Information from the journal.
Communications
Figure 2. EDS data of an iridium film grown from (hfac)IrI(1,5-COD) on a SiO2 substrate at 350 °C.
silicon (SiO2/Si), Pt-coated silicon (Pt/Si), and Cu-coated silicon (Cu/Si) wafers were used as substrates. Deposition experiments were conducted at substrate temperatures over a range from 250 to 400 °C and at precursor bubbler temperatures from 60 to 65 °C in a warm-wall bell jar CVD reactor. N2 was used as carrier gas and 5% H2 in N2 as a reactive gas. The reactor pressure was maintained between 1 and 4 Torr during the experiments. Under these conditions, reflective Ir films were deposited on SiO2/Si, Pt/Si, and Cu/Si substrates. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), and energy dispersive spectroscopy (EDS). The XRD data revealed that the film was composed of iridium metal. XRF and EDS (see Figure 2) data further confirmed the presence of the Ir in the film. The morphology of a film grown on SiO2/Si at a substrate temperature of 350 °C was investigated by SEM, which shows a rough texture with grain size ranging from 0.1 to 0.3 µm.20 A typical deposition rate was calculated to be 150 Å/min at a substrate temperature of 350 °C and a precursor bubbler temperature of
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65 °C. The film resistivity was measured using a fourpoint probe and found to be 45 µΩ cm at 20 °C. For comparison, a sputtered Ir film21 was deposited and found to be 22 µΩ cm at 20 °C using the same equipment. Therefore, the relative film resistivity is 2 times that of sputtered bulk films. In summary, the synthesis and characterization of (hfac)IrI(1,5-COD) and (thd)IrI(1,5-COD) were described. The (hfac)IrI(1,5-COD) was structurally characterized for the first time by single crystal X-ray diffraction. Using these new precursors, Ir films were deposited by CVD at substrate temperatures between 250 and 350 °C in the presence of hydrogen. The advantages of these compounds are the ease of synthesis and handling, excellent volatility and thermal stability, and the formation of Ir films at relatively low temperatures. Detailed studies related to the CVD of both Ir and IrO2 films, via both a conventional precursor delivery and a liquid-source delivery using these new precursors, are in progress. Acknowledgment. The authors thank ATMI for financial support and Drs. Jeff Roeder and Bryan Hendrix for invaluable discussion. Supporting Information Available: The thermal analysis data of (hfac)IrI(1,5-COD), scanning electron micrograph of the deposited Ir film, X-ray structure determination summaries and tables of bond lengths and angles, anistropic displacement coefficients, and H atom coordinates (19 pages). See any current masthead page for ordering information. CM980346X (21) A sputtered thin-film of iridium was deposited using a Varian M2I deposition system and used to calibrate the four-point probe electrical test equipment.
